1. Field of the Invention
The present invention generally relates to an image forming apparatus, such as a copying machine, a printer, or a facsimile, which adopts an electrophotographic printing method, and more particularly to a color image forming apparatus including an intermediate transfer member cleaning apparatus for cleaning an intermediate transfer member on which a multi-color image is formed by superimposedly transferring images formed on image bearing members.
2. Related Background Art
In recent years, as an image forming apparatus that forms an image using an electrophotographic printing method, for instance, an image forming apparatus using an intermediate transfer member that first forms a multi-color image by superimposedly transferring images formed on image bearing members onto the intermediate transfer member and then transfers the multi-color image from the intermediate transfer member to a recording material has been widely used because of needs for formation of high-quality images on various kinds of paper as recording materials.
As the intermediate transfer member, an intermediate transfer belt is widely used. Also, as the intermediate transfer belt, a belt made of a resin generally represented by polyimide or the like is widely used because of its characteristics of realizing high image quality, long life span, and high stability. Further, as belt cleaning means for cleaning the intermediate transfer belt after transfer of a multi-color image onto a recording material, a blade method disclosed in JP 2001-305878 A is widely used. Considering the surface property of the resin belt and the like, the cleaning means based on the blade method has a high cleaning capability.
Meanwhile, recently, in order to further improve image quality and stabilize the cleaning capability of the cleaning means based on the blade method, for instance, diameters of developers (toner) have been reduced and shapes of the toners have been changed into nonspherical shapes.
As a result of the changes of the toner, however, with the intermediate transfer belt made of a resin, a problematic hollow characters phenomenon occurs at the time of transfer. The hollow characters phenomenon is a phenomenon in which at the time of transfer of an image, toner deformation by stress occurs due to application of a high pressure to the image and therefore a cohesive force between toners is increased and a part of the image is not transferred and remain on an image bearing member. The phenomenon occurs particularly conspicuously in the case of transferring letters, line images, and the like. In the case of the resin belt, the pressure applied to images at the time of transfer is very high, so the hollow characters phenomenon becomes particularly problematic.
Therefore, in recent years, in order to solve the hollow characters problem, an elastic intermediate transfer belt having a layer structure including at least one elastic layer has become a mainstream in place of the intermediate transfer belt formed by using a resin.
It is known that the elastic intermediate transfer belt is effective in solving the hollow characters problem because it includes at least one elastic layer in its layer construction and therefore is soft and is capable of reducing a pressure exerted on toner at a transfer portion. It is also known that the elastic intermediate transfer belt is effective not only in improving transfer efficiency with respect to general paper but also in improving a transfer property with respect to thick paper and a transfer property with respect to paper having projections and depressions on its surface due to its superior adhesiveness with paper as a recording material at a secondary transfer portion.
However, when the blade method described above is used to clean the elastic intermediate transfer belt, a contact load of the cleaning blade with respect to the elastic intermediate transfer belt is increased due to elasticity of the surface layer of the elastic intermediate transfer belt, the tip of the edge of the cleaning blade bites into the belt surface layer, and behavior of the tip of the edge of the cleaning blade becomes unstable, which leads to a cleaning failure. In addition, there is a fear that a problem such as a wire edge, flutter, or noise of the cleaning blade, and an inconvenience, such as a flaw in the elastic belt surface layer, or toner fusion bond will occur due to an increased frictional force between the belt and the cleaning blade, which lowers image quality.
Therefore, in recent years, in order to circumvent the inconvenience described above, an electrostatic fur brush with a less contact load with respect to the elastic intermediate transfer belt has been generally used as the cleaning means for cleaning the elastic intermediate transfer belt.
For instance, there is an electrostatic fur brush method described in JP 3236442 B with which a cylindrical member obtained by winding a conductive fiber around a metal core is abutted to a belt under a state in which a bias is applied, and a bias whose polarity is opposite to the polarity of toner is applied, thereby electrostatically attracting the toner with a fur brush and removing the toner from an image bearing member.
It is known that as compared with the blade method, with which toner is mechanically removed, the fur brush method, with which toner is electrostatically attracted and is removed from a belt, has limitations on a cleanable toner amount and toner polarity. With the electrostatic fur brush method, it is impossible to achieve the inherent effect of the fur brush unless toner is electrostatically attracted by the fur brush and then is further transferred from the fur brush using a flicker, a bias applying roller, or the like, so when the amount of toner attracted by the fur brush increases, cleaning performance deteriorates, which means that the electrostatic fur brush method is generally inferior to the blade method in terms of cleanable amount.
Also, as described above, the fur brush method is a method with which toner is cleaned through attraction by a fur brush, so only toner having a polarity that is opposite to the polarity of a bias applied to the fur brush is cleaned.
However, depending on the value of the bias applied at the time of transfer, there is a case where the polarity of transfer residual toner that remains on an intermediate transfer belt after transfer of a toner image to paper is reversed from positive to negative or from negative to positive. The transfer residual toner, whose polarity has been reversed, has the same polarity as the bias applied to the fur brush, so the toner is not attracted by the fur brush and passes through the fur brush. The toner having passed through the fur brush is superimposed on the next image, which may cause an image defect. Therefore, as disclosed in JP 2002-207403 A, two fur brushes are used as cleaning means and biases having different polarities are respectively applied to the fur brushes, thereby making it possible to attract and remove toner with the fur brushes with reliability regardless of the polarity (either positive or negative) to which the toner has been charged due to a bias applied at a secondary transfer portion, a use environment, toner degradation, and the like.
However, in the image forming apparatus described above, when a toner image on an intermediate transfer member is not appropriately transferred to a recording material due to occurrence of a paper jam, a large amount of toner remains on the intermediate transfer member. In order to remove the large amount of toner, it is required to pass the residual toner through the cleaning means multiple times, which results in the necessity to rotate the intermediate transfer member multiple times.
Consequently, when a toner image is not appropriately transferred to a recording material and remains on an intermediate transfer member, a long time is required to remove the toner from the intermediate transfer member, which leads to a problem in that the period of time for which image formation cannot be performed by an image forming apparatus increases.